Manufacturers of frequency converters can utilize various techniques in controlling the behavior of the torque of a motor with respect to a rotational speed of the motor in an electric drive application.
The applications can, for example, be divided into two groups on the basis of the behavior of the load: linear torque/speed ratio applications and quadratic torque/speed ratio applications. In linear (torque/speed ratio) applications, the torque applied to the load is directly proportional to the rotational speed. In quadratic (torque/speed ratio) applications, the torque is proportional to the square of the rotational speed.
Some linear applications, such as constant-torque loads found in industrial applications, can require high dynamic performance. In order to be able to maintain a full torque output from the motor at various motor speeds, the drive provides the motor with a nominal flux.
However, in some quadratic applications, such as pump or fan applications, the dynamic performance requirements cannot be as demanding as in linear applications. In such applications, the flux applicable by the drive can be limited, thus allowing more economic performance. On the other hand, this approach can result in a reduced dynamic performance of the drive, as there is a more limited flux capability available than with the nominal flux.
In some frequency converters, one of the above approaches, i.e. a more dynamic performance or a more economic performance, can be selected as the default performance approach, and the other can be selected by the user. The user does not, however, always select the more appropriate approach for the application in question.